The present invention relates generally to vertical deflection circuits for camera tubes in television cameras, and more particularly to a circuit for carrying out vertical deflection so as to carry out DC restoration of black level of a video signal irrespective of amplitude fluctuations of a vertical deflection voltage.
In general, a dark current exists in the camera tube of a color television camera. This dark current is not continually constant but fluctuates or becomes irregular due to temperature variation. When there is a fluctuation in this dark current, the white balance is destroyed. Particularly in a frequency separation system in which an optical stripe filter is used, the green color signal transmitted at low frequency is directly influenced by the dark current.
Heretofore, in color television cameras of the single-tube or two-tube type of a color multiplex system in which vidicon camera tubes are used as camera tubes for chrominance signals, optical filters provided with optically black parts vertically at the end portions in the horizontal scanning direction of the effective picture have been used. A video signal obtained from a camera tube by using an optical filter of this character has a black level portion produced by the optically black part in the trailing edge of the horizontal beam blanking of each horizontal scanning period. Heretofore, correction for black level fluctuation due to dark current fluctuation has been carried out by clamping this black level portion, whereby the DC restoration has been carried out.
However, when there is a flaw or damage in the optically black part of the optical filter, or when there is a flaw in the photoconductive film or nesa film of the camera tube corresponding to this black part, a signal of high level in pulse form is generated in the signal part corresponding to the optically black part in the image pickup signal. In the above mentioned known system, however, when the unwanted pulse signal due to a flaw is generated, clamping occurs with this unwanted signal as a reference, and accurate black level clamping cannot be carried out. Furthermore, the dark current level, in general, is not uniform over the entire photoconductive surface of the camera tube in the horizontal scanning direction but is higher at the two end parts than the central part. (This level distribution will hereinafter be referred to as "dark current shading".) By the above mentioned known system, the effect of this dark current shading could not be reduced.
Another example of a color television camera is that wherein use is made of an optical filter comprising a color stripe filter provided on the upper or lower portion thereof laterally with an optically black part. In this color television camera, the DC restoration is generally carried out by clamping a black level output signal part in the output signal of the camera tube by a clamping pulse.
When there exists a variation in environmental temperature, the amplitude of the saw-tooth voltage to be applied to a vertical deflection coil undergoes change, whereby the vertical deflection width thus changes.
A vertical deflection circuit known heretofore has been arranged so that a saw-tooth voltage for vertical deflection is supplied by way of a capacitor to a vertical deflection coil. In this known circuit, the average level of the saw-tooth voltage is adapted to correspond to the center of vertical deflection in the camera tube, and deflection is carried out upward and downward with respect to the above described center. Accordingly, as will be described in conjunction with drawings hereinafter, when the amplitude of the saw-tooth voltage fluctuates due to causes such a temperature fluctuation, there occurs fluctuation in a position of the black level output signal part corresponding to the black part of the optical filter, among the output signal of the camera tube.
The clamping pulse generated from an electrical circuitry for clamping the output black level signal for DC restoration has a timing for clamping the black level in a case where the saw-tooth voltage has a normal amplitude thereby carrying out vertical deflection normally, and accordingly the output black level signal is positioned at the normal position. Heretofore, position of the output black level signal undergoes fluctuation due to causes such a temperature fluctuation, as described above. For this reason, there give rise to difficulties that clamping cannot be carried out appropriately, which results in improper DC restoration of black level.
For getting rid of the above described effect of dark current shading and for causing the black level signal not to appear in the final video signal, it is required to narrow the width of black part of the optical filter, thus shortening a period of time when the output black level signal exists. However, this arrangement involves difficulty that the accurate DC restoration of the black level cannot carried out under even slightest position fluctuation, because the period of time when the output black level signal exists becomes short.
Accordingly, a manner for correcting electrically the amplitude of the saw-tooth voltage so that it does not fluctuates irrespective of temperature fluctuation may be conceivable. However, a circuit arrangement according to this manner will be extremely complex, whereby the television camera becomes expensive and bulky. Thus, the above manner is not suitable for potable type television cameras which are preferred to be inexpensive and of small size.